In the semiconductor manufacturing industry, considerable progress has been made in development of higher-level integration of devices. Particularly, in manufacture of ULSIs, fine patterns of 1 .mu.m or less are demanded.
During the ULSI manufacturing process, adhesion of fine dusts or a trace amount of impurity ions to wiring patterns may cause a circuitry problem such as a short circuit, resulting in lower yield of the product (i.e., the ULSI device). Therefore, fluid such as gas or water used in the ULSI manufacturing process must be of high purity. In other words, the amount of particles or impurities in the fluid must be very small. Accordingly, when piping or a piping member (hereinafter may be referred to as "piping") is used to supply the high-purity fluid, release of particles, molecules, ions, or impurities from the inner surface thereof--the surface with which the fluid comes in contact--must be suppressed to the extent possible.
Typical materials of the above-described piping include austenitic stainless steel, inter alia, SUS316L. Depending on the specific application, austenitic-ferritic dual phase stainless steel and ferritic stainless steel may also be used.
The inner surface of a stainless steel pipe used for the above-described purposes is smoothed so as to prevent generation of dusts and so as to prevent impurities from adhering to or being absorbed onto the inner surface. Specifically, the inner surface undergoes cold drawing or electro-polishing so as to reduce to the extent possible the surface area which comes into contact with the high-purity fluid.
However, when the high-purity fluid is a corrosive gas such as chlorine gas, hydrogen chloride gas, or hydrogen bromide gas, or a chemically unstable gas such as a silane gas; in other words, when the high-purity fluid is a "special material gas," the inner-surface-smoothing treatment alone is not sufficient. That is, when the special material gas to be employed is a corrosive gas such as chlorine gas, hydrogen chloride gas, or hydrogen bromide gas, the stainless steel pipe must be corrosion resistant. In contrast, when a chemically unstable gas such as a silane gas is used as the special material gas, the stainless steel pipe must have non-catalytic characteristics (in other words, the pipe must not exhibit catalytic properties which decompose the gas into a fine particulate substance when the inner surface of the pipe comes into contact with, for example, a silane gas).
It has been reported that such properties can be improved by heating stainless steel in an atmosphere adjusted to have a low oxygen partial pressure to thereby form Cr oxide film on the surface of the stainless steel (see "Non-corrosive, Non-catalytic Cr.sub.2 O.sub.3 Stainless Steel Piping Technique for Special Gases," the 24th ULSI Ultra Clean Technology Workshop Proceedings, p. 55-67; Jun. 5, 1993; sponsored by Institute of Basic Semiconductor Technology Development). Since the steel referred to in this article contains about 15 at. % Cr and about 15 at. % Ni, it is austenitic stainless steel, and is presumably SUS316L.
Japanese Patent Application Laid-Open (kokai) Nos. 7-197206 and 7-233476 disclose methods of forming Cr oxide film on the surface of a stainless steel. Specifically, kokai publication No. 7-197206 discloses a method of forming Cr oxide film on the surface of a dual phase stainless steel having a heavily deformed microcrystalline zone, and kokai publication No. 7-233476 discloses a method of forming Cr oxide film on the surface of a ferritic stainless steel. In addition, Japanese Patent Application Laid-Open (kokai) No. 8-302448 discloses a method in which Cr oxide film is formed on the surface of a ferritic stainless steel such that the film has a thickness of 7-50 nm and the grains thereof, which contain Cr in an amount of 90 at. % or more based on the total amount of the constituent elements other than oxygen, have a diameter of 200 nm or less.
However, neither of these publications discloses a method for forming Cr oxide film on the inner surface of a stainless steel pipe which has a length as long as four meters and which is often used as a piping member in a semiconductor manufacturing process, such that Cr content and film thickness are uniform over the entire length of the inner surface.
Japanese Patent Application Laid-Open (kokal) Nos. 2-43353 and 3-111552 disclose techniques for forming oxide film on the inner surface of a stainless steel pipe. Specifically, kokai publication No. 2-43353 discloses a metal oxidation apparatus and a metal oxidation method, and kokai publication No. 3-111552 discloses an apparatus for oxidation treatment of metallic pipe. The techniques disclosed in these publications are of batch-type processing, in which steel pipe is heated from the outer surface while the pipe is secured in a heating furnace and a gas having a specific composition is fed through the steel pipe so as to oxidize the inner surface of the steel pipe in a specific atmosphere at a specific temperature.
When a stainless steel pipe having, for example, a diameter of 6.35 mm and a length of 4 m, which are the dimensions of a piping member often used in a semiconductor manufacturing process, is oxidized by the batch-type processing method, the process is cumbersome and has very poor efficiency. Specifically, the following steps must be repeated for each batch: a steel pipe to be treated is placed in a heating furnace; an inert gas is fed into the pipe in order to purge residual air from the pipe and heating furnace; the pipe is heated for removal of moisture from its inner surface (a so-called baking treatment); supply of the inert gas used for purging the residual air in the steel pipe is switched to supply of a gas for oxidation treatment; and the heating furnace is operated in order to heat the steel pipe for oxidation, after which the furnace is cooled and the steel pipe is removed from the furnace.
In order to improve the efficiency of the batch-type processing and to perform the oxidation treatment on a plurality of steel pipes at one time, the capacity of the heating furnace must be increased. Additionally, in order to heat a plurality of the steel pipes uniformly, a control device must be installed in the heating furnace. Accordingly, the cost of the equipment increases, which results in problems in terms of economy. In addition, in the batch-type processing, only the longitudinal central portion of a steel pipe is heated uniformly. In order to heat a steel pipe as long as 4 m along its entire length, the heating furnace must have a very large capacity and a very long steel pipe holder, which again results in problems in terms of economy.
Moreover, the conventional batch-type processing fails to provide Cr oxide film having a uniform Cr content and uniform thickness along the entire length of the steel pipe. This is because oxidation reaction occurs simultaneously along the entire length of the pipe; one end of the pipe from which an oxidizing gas is fed is most likely to be oxidized, while the other end from which the oxidizing gas is discharged is difficult to oxidize, resulting in a nonuniform Cr oxide film. The gas-exit end is not easily oxidized, because the concentration of the oxidizing substance; i.e., steam and oxygen in an oxidizing gas, becomes low at the gas exit end as compared to the feeding end.
In the heat treatment, another technique is used, in which steel pipes are moved so that the heat treatment is performed on the steel pipe in a continuous process. A technique of such continuous heat treatment applied to stainless steel pipes is generally known as bright annealing. In the case of bright annealing, the heat treatment is performed such that a metallic luster of stainless steel pipe is maintained while oxidation of both the inner and outer surfaces of stainless steel pipe is prevented. Bright annealing is conducted in order to remove the skewness of steel pipe caused by cold working such as cold drawing and to recrystallize the structure of the metal.
Bright annealing can be performed in a reducing atmosphere while the pipe is moved in a pipe-length direction. In this case, a reducing gas, such as hydrogen gas or a mixture of an inert gas and hydrogen gas, is fed into the inside of the steel pipe, while the pipe is moved in a pipe-length direction and inserted into a heating furnace whose atmosphere is also controlled to contain a reducing gas. In the case of the bright annealing, measures are taken to prevent oxidation of both the inner and outer surfaces of the steel pipe. Thus, both the inner and outer surfaces of the stainless steel pipe are exposed to the reducing gas atmosphere. Mixing the gas applied to the inner surface of the pipe with the gas applied to the outer surface presents no problem.
However, in the case of stainless steel pipe used in a semiconductor manufacturing process, only the inner surface of the pipe must be oxidized. Therefore, the above-described continuous method of bright annealing cannot be applied to oxidization of the inner surface of the stainless steel pipe used in a semiconductor manufacturing process.